Growth factors signaling through the cell-surface receptor tyrosine kinases (RTKs) represent one of the most critical pathways in the cellular and inter-cellular regulation of vertebrate biological systems. Defects/malfunctions of the class III RTKs, such as the Stem Cell Factor (SCF) receptor KIT, the Macrophage Colony Stimulating Factor (MCSF) receptor FMS, and the FLT3 Ligand (FLT3L) receptor FLT3, contribute to the genesis and development of many types of cancers such as leukemia, gastrointestinal stromal tumor (GIST), and others. These receptors are ideal targets for cancer therapies, but the mechanistic basis underlying their ligand recognition and activation is incompletely understood. Our long-term goal is to elucidate the structural mechanisms used by these receptors in cellular regulation and malignancies. The project will perform a thorough analysis of the whole activation process of class III RTKs: how these receptors maintain static states when the ligands are not present, how their extra cellular domains recognize ligands, and what are the conformational changes required for receptor activation. The specific aims are: (1) Biochemical reconstitution and structural analysis of the complex between KIT and its ligand SCF. Crystals of the SCF/KIT complex that diffract X-rays to 3.2 Angstroms resolution have been obtained. The structure will be solved by multi-wavelength anomalous diffraction (MAD). (2) Biochemical reconstitution and structural analysis of the complex between FMS and its ligand MCSF. Crystals of the MCSF/FMS complex that diffract to 2.4 Angstroms resolution have been obtained. The structure was determined by a combination of partial molecular replacement and single isomorphous replacement with anomalous scattering (SIRAS); refinement is in progress. (3) Biochemical characterization and structural analysis of the complex between FLT3 and its ligand FLT3L. Small crystals of the FLT3L/FLT3 complex have been obtained recently. (4) Investigation of the binding specificity and energetics of the receptor/ligand interactions through protein engineering of the receptor/ligand interfaces. These studies will elucidate novel structural mechanisms that will lay the groundwork for therapeutic development to treat class III-RTK-related cancers. [unreadable] [unreadable] [unreadable]